In radio reception and radio transmission arrangements, “AGCs” (Automatic Gain Control) are normally used to control the amplitude gain for the purpose of automatic gain control in the respective signal paths. Such AGCs use continuous control for the amplitude gain and are predominantly used, by way of example, in mobile radio transceivers which operate on the basis of the GSM (Global System for Mobile Communication) standard.
By contrast, the recently introduced third-generation mobile radios have greater demands on the gain range and on the accuracy of the gain setting. This is because these communication appliances use code division multiple access (CDMA), whereas previously time division multiple access (TDMA) or frequency division multiple access (FDMA) have normally been used. Since the modern mobile radio methods involve some of the information being transmitted in the modulation of the amplitude, much greater demands on the gain control arise overall. In addition, the AGC not only needs to be carried out with greater accuracy and with a larger setting range but also needs to be performed continuously both in the transmission mode and in the reception mode.
In transmission arrangements based on UMTS (Universal Mobile Telecommunications Standard), the maximum output power for a class 3 transmitter is specified as +24 dBmW and the minimum output power is specified as −50 dBmW, for example. Accordingly, the required power control range of the transmitter is at least 74 dB in magnitude. To these 74 dB, it is necessary to add the variation in the gain over the amplifier signal chain of at least 6 dB. It is accordingly necessary to be able to cover a minimum gain range of 80 dB in practice.
The specification also demands that it is necessary to adhere to the gain in stages of 1 dB with an accuracy of +/−0.5 dB for all temperatures, process tolerances and frequencies.
In addition, with respect to the lowest possible power consumption, provision should be made for the ten stages with the highest gain from the total of 80 amplifier stages to have an accuracy of 0.1 dB in preference. In a receiver based on UMTS, the input signal may assume any level between −99 dBm and −25 dBm, for example. This results in a gain range to be covered, including tolerance compensation, of at least 80 dB. Normally, this results in a gain of 20 dB in one stage from the input-side, low-noise amplifier (LNA), while in baseband the remaining 60 dB are covered in stages of 1 dB.
The document DE 101 63 466 A1 specifies a transmission arrangement for continuous-time data transmission where programmable amplifiers are provided in baseband and/or in the radio-frequency path of the signal processing chain for the purpose of signal amplification.
In contrast to the AGC, a programmable gain control (PGC) is understood in the present instance to be adjustable in discrete steps of the amplitude gain.
Transceiver architectures which meet multi-system, multi-frequency-band and multi-mode requirements and can be integrated on a large scale are normally designed as transmitters with a direct-conversion architecture and receivers with a direct-conversion architecture. The fact that such transceivers do not perform any intermediate-frequency processing means that it is necessary to distribute the gain control over the baseband signal processing and the radio-frequency signal processing.
In UMTS systems, baseband covers a frequency range from 0 Hz to 1.92 MHz, for example. The radio-frequency range in the case of UMTS systems means a frequency band from 1920 to 1980 MHz in the transmitter and from 2110 to 2170 MHz in the receiver, for example.
At present, continuous gain control (AGC) is preferably used in transmission arrangements based on code-division multiple access, in order to meet the requirements regarding error vector magnitude (EVM) and the transmission-spectrum mask over the entire dynamic range. In addition, the gain control cannot respectively be effected before the actual transmission timeslots, as in the case of time-division multiple access systems, but rather needs to be performed during the continuous user-data transmission.
Although the programmable gain control (PGC) affords advantages with respect to lower power consumption, smaller chip area, reduced number of pins, greater robustness toward radiated interference and higher flexibility, and also permits greater accuracy at lower cost, a few problems may still arise with the programmable gain control. If, by way of example, both gain control blocks with 1-dB stages and gain control blocks with 6-dB stages are used in baseband of the receiver, then offsets may arise for the gain, particularly when the gain changes from one block to the other. In addition, it is a complex matter to ensure exactly 20 dB of gain in the receiver's LNA regardless of the frequency range which is set, the temperature and process tolerances.
In the transmission chain, the radio-frequency part normally contains a gain block with 6-dB stages, whereas baseband contains 1-dB stages. In this case too, problems may arise with the accuracy of the 6-dB amplifier stages with respect to process fluctuations, temperature dependencies and frequency changes. The accuracy problems described arise particularly when changing from one block to another. By way of example, when changing a gain of 5 dB, which is provided using an amplifier with a step size of 1 dB, to a gain of 6 dB, which is provided in an amplifier with a step size of 6 dB, the accuracy problems described may arise.